A mathematical simulation model is conceptually the ideal tool to decide which pollutant loads to treat first, given limited funds, or to determine the admissible loads to a given water body. The model is indeed a tool that allows to explore the effects of hypothetical actions on water quality prior to their implementation. Unfortunately, the application of ‘orthodox models’, i.e. those based on mass balance and physical laws, faces difficulties so harsh to prevent their real utilisation, at least at the wide-area level, with feasible time and costs. But not everything is lost. In this paper a very simple model is developed which borrows from physics a very basic principle: ‘if there is no load, water is clean’, and assumes that in each given water body there exists a linear relationship between load reduction and water quality improvement (but not necessarily between load and quality in the overall system); an assumption generally acceptable from the practical point of view. The rest is borrowed from logic, this is why I named it ‘quali-logical’. This model can provide answers to the questions raised above. It assigns to the measured water-quality-state a key role (contrary to orthodox models that use it basically in model calibration only), yet it does not require to comply with strict data collection protocols as imposed when modelling is the aim. It does not require to quantify pollutant loads, but just to (qualitatively) assess how the total load is partitioned amongst: load carried by the upper reach, the possible tributary, non-point sources, and point-wise sources. It does not require the knowledge of water flows. It even does not need calibration, although this can be carried out. And it does not assume that water quality compounds are conservative. Being very simple, it can be easily implemented.